Reworking processes during monogenetic eruptions. The case of the Parícutin volcano

One of the best-known examples worldwide of monogenetic volcanism is the Parícutin volcano. The eruption began its formation in the middle of a cornfield in February 1943 and lasted until March 1952. Parícutin is the youngest edifice of the Michoacán-Guanajuato Volcanic Field, which was witness initially by local inhabitants, and later by scientists and other observers. Observations of the eruption documented the remobilization of primary ashfall by rainfall and wind. Despite these observations, the resulting reworked deposits have not yet been described in the stratigraphic sequence. The distinction between primary pyroclastic and reworked deposits is critical for the geological understanding of eruptive processes and related hazards because of their different origins, frequencies, and environmental impacts. This categorization is not always obvious and needs a detailed study to characterize the complex interbedding of both types of deposits that coexist in the volcanic sequence. Referenced to these, we conducted new field reconnaissance, coupled with laboratory analyses of the ejecta ash fraction. The detailed composite stratigraphy obtained consists of six widely dispersed fallout deposits interbedded with seven reworked units. These reworked deposits display sedimentary structures produced by tephra remobilization due to lahars and stream flows. In addition, some layers show dunes and ripples generated by duststorms. By using GIS tools, we integrated the existing data with our new composite stratigraphic column and the distribution map of the syn-eruptive reworked deposits. This analysis reveals that more than 70% of the total thicknesses correspond to syn-eruptive reworked deposits. Therefore, previous studies had overestimated the distribution of primary tephra from the Parícutin explosive phases. The lowest and flattest areas with wide rill networks, which are located 4 to 6 km north of the volcano, are composed of up to 90% reworked deposits. In contrast, proximal locations with gentler slopes located at medium altitudes better preserve pyroclastic deposits. To that end, we constructed a new isopach map of the pyroclastic deposits based on the distribution of the reworked deposits. This study brings new light to understanding the sedimentary processes that occur during volcanic eruptions and highlights the importance of recognizing pyroclastic and reworked deposits during monogenetic eruptions.